1. Field of the Invention
This invention relates to transportable hydraulic seismic transducers and more particularly to the seismic energy source attached to a transport and to a pad for imparting elastic waves to the underlying ground.
2. Description of the Prior Art
In the prior art, the conventional hydraulic actuator of a seismic vibrator consists of several parts. The reaction mass is a main element which has a double acting hydraulic actuator housed inside. In addition, a reaction mass is used as an inertial element for the developed force to act upon. The reaction mass is machined internally to house a piston, piston rods, a liner and rod bushings. Also, seals at the end of the actuator and piston rings are required.
The piston rods run on the rod bushings which provide a bearing surface. The bearing surface is lubricated by hydraulic fluid passing from the actuator cavity to the rod seal area.
The pressure on opposite faces of the piston are sealed from each other by the piston rings. The rings form a very low leakage path by slidably sealing the gap between the inner diameter of the liner and piston outer diameter.
The liner is a sacrifical surface on which the piston rings slide. When the rings of the liner are worn, they are replaced. The liner is shrunk fit (liquid nitrogen) into a machined hole in the reaction mass and then bored to its proper inner diameter.
The reaction mass has several different internal machined features. From both ends, a desired diameter bore is machined. Then a second diameter bore, generally smaller, which contains the liner and the piston is machined to connect the bores at the ends. It is of the utmost importance that these three bores have the same center. It is essentially impossible to achieve that ideal and the deviation must be accounted for in the mating parts. Also, the mating parts which have different diameters suffer the same problem.
The concentricity of the three bores is required to be less than some specified upper bound. To insure proper performance, the tolerances and the concentricities of the parts are specified and the resultant fabrication costs are quite high.
By using a single diameter bore, this invention does away with a great deal of the expense and inherent inaccuracies of the above described prior art.